Solution doping method of making an optical amplifying fiber

ABSTRACT

An optical amplifying fiber including a clad, a first core provided inside the clad and containing Ge, a second core provided inside the first core and containing Er and Al, and a third core provided inside the second core and containing Ge. The second core has a refractive index higher than that of the clad, and the first and third cores have refractive indexes each of which is higher than that of the second core. Since the third core having the high refractive index is provided at a central portion, it is possible to make smaller a mode field diameter and hence to improve a conversion efficiency of pumping light into signal light. Further, since the second core contains Al as an amplification band width increasing element, it is possible to sufficiently ensure a wide amplification band width.

This application is a divisional of application Ser. No. 08/868,397,filed Jun. 3, 1997, now allowed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber applicable to anoptical fiber amplifier and a process of producing the optical fiber.

2. Description of the Related Art

An optical amplifier capable of directly amplifying light signalswithout conversion into electric signals is advantageous in that it canbe easily enlarged in capacity because of a substantially bit rate freefunction thereof and that it can collectively amplify multiple channels,and from this standpoint, it is being extensively studied as one of keydevices of future optical communication systems by various researchorganizations.

There is known one form of an optical amplifier using a single modeoptical fiber including a core doped with a rare earth element such asEr, Nd, or Yb (hereinafter, referred to as “a doped fiber”), whereinsignal light to be amplified is transmitted to the doped fiber and atthe same time pumping light is introduced into the doped fiber in thedirection identical or reversed to that of the signal light.

The optical amplifier using the doped fiber, which is called an opticalfiber amplifier, has excellent features of eliminating a polarizationdependency of a gain, lowering a noise, reducing a loss in coupling withan optical transmission path, and the like. In practical use of theoptical fiber amplifier of this type, it is required to make wider awavelength band width of signal light in which the signal light can beamplified at a specific gain (hereinafter, referred to simply as “awavelength band width”) and to make higher a conversion efficiency ofpumping light into signal light.

As for light having a wavelength within a range of 0.8 to 1.6 μm, therehave been established a technique of producing an optical fiber using aquartz glass suitable for long-distance transmission, and a technique ofputting the optical fiber into practice. An optical fiber is obtained bydrawing a preform in the shape of a thick rod. The preform is requiredto have a composition gradient in a cross-sectional direction thereofwhich is accurately set as designed.

A standard process of preparing a preform has been known, in which aglass composition chemically converted from reactive gases is depositedon an inner surface of a quartz reaction tube by a MCVD (Metal ChemicalVapor Deposition) process or the like. In the MCVD process, suitablereactive gases such as SiCl₄ and O₂ are introduced in the quartzreaction tube, and the quartz reaction tube is heated at a temperaturesuitable for reaction of the gases. A heating zone is moved in thelongitudinal direction of the quartz reaction tube, to deposit a newglass layer on an inner wall surface of the quartz reaction tube. Aplurality of layers, for example, 20-30 layers are repeatedly deposited.A composition gradient in the cross-sectional direction of an opticalfiber produced from the preform can be controlled by independentlyadjusting compositions of the layers of the preform. After the layersare fully deposited, the quartz reaction tube is collapsed by heating,to be thus formed into a rod-shaped preform. The preform is then drawnto produce an optical fiber.

In the MCVD process, a reactive material vaporized at a room temperatureis generally used. For example, SiCl₄ is used for forming SiO₂ which isa main component of an optical fiber, and GeCl₄ is used for forming GeO₂which is an element for adjustment of a refractive index. Incidentally,for production of a doped fiber, a suitable reactive material containinga rare earth element sufficiently evaporated at a room temperaturecannot be obtained, differently from SiCl₄ and GeCl₄, and consequently arare earth element cannot be doped in the doped fiber at a practicallysufficient concentration only by the MCVD process. For this reason, arare earth element has been doped in a doped fiber at a practicallysufficient concentration in the following manner.

A known process of preparing a preform suitable for production of adoped fiber includes a step (1) of depositing a soot-like core glass onan inner surface of a quartz reaction tube, a step (2) of allowing thesoot-like core glass impregnated with a solution containing a rare earthelement compound as a solute, and a step (3) of drying the solution andcollapsing the quartz reaction tube. On the other hand, a technique ofwidening a wavelength band width of an optical fiber amplifier using adoped fiber has been proposed, in which a core is impregnated with Al₂O₃as well as a rare earth element.

For example, Japanese Patent Laid-open No. Hei 5-119222 discloses adouble core structure including an aluminum/silica based glass(Er-Al-SiO₂) doped with erbium (Er) and aluminum (Al), which is providedat a center portion of a core; and a germanium/silica based glass(Ge-SiO₂) doped with germanium (Ge), which is provided at an outerperipheral portion of the core. In the prior art structure disclosed inJapanese Patent Laid-open No. Hei 5-119222, however, is disadvantageousin that a relative index difference Al of the core peripheral portion isabout 2% but a relative index difference Δ2 of the core center portionis about 0.7% at the utmost, with a result that there occurs a largedepression of a refractive index at the core central portion.

This is due to the fact that an element doped for widening a band width,such as Al, acts to decrease a refractive index. The depression of arefractive index causes a phenomenon in which a mode field oftransmission light is spread and thereby a mode field diameter is madelarger. The mode field diameter thus increased is inconvenient inconverting pumping light into signal light and results in degradation ofa conversion efficiency of pumping light into signal light. For example,in the prior art structure disclosed in Japanese Patent Laid-open No.Hei 5-119222, a mode field diameter was about 4.8 μm and a conversionefficiency of pumping light into signal light was 64%.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalamplifying fiber capable of widening a wavelength band width, andimproving a conversion efficiency of pumping light into signal light bysuppressing a mode field diameter at a small value, and to provide aprocess of producing the optical amplifying fiber.

In accordance with an aspect of the present invention, there is providedan optical amplifying fiber including: a clad having a first refractiveindex; a first core, provided inside the clad, containing a refractiveindex increasing element and having a second refractive index higherthan the first refractive index; a second core, provided inside thefirst core, containing a rare earth element and an amplification bandwidth widening element and having a third refractive index higher thanthe first refractive index and lower than the second refractive index;and a third core, provided inside the second core, containing arefractive index increasing element and having a fourth refractive indexhigher than the third refractive index.

The refractive index increasing element may be selected from Ge and Ti.Of these elements, Ge is preferably used. The second core preferablycontains Er and Al.

According to another aspect of the present invention, there is provideda process of producing an optical amplifying fiber, including the stepsof: (a) forming a first core layer mainly made of SiO₂ doped with GeO₂or TiO₂ on an inner surface of a quartz reaction tube by chemical vapordeposition; (b) forming a soot-like second core layer mainly made ofSiO₂ on the first core layer by chemical vapor deposition; (c) allowingthe second core layer impregnated with a solution containing a rareearth element and at least one element selected from a group consistingof Al, Zn, Sn and La; (d) evaporating a solvent of the solutionimpregnated in the second core layer; (e) heating the second core layerto vitrify the second core layer; (f) forming a third core layer mainlymade of SiO₂ doped with GeO₂ or TiO₂ on the second core layer bychemical vapor deposition; (g) perfectly collapsing the quartz reactiontube by heating to form a preform; and (h) melting and spinning thepreform.

In the optical amplifying fiber of the present invention, since thethird core having a large relative index difference is provided at thecore central portion, a light power is concentrated at the core centralportion as compared with the doped fiber having the prior art structure,to make smaller a mode field diameter, thereby improving a conversionefficiency of pumping light into signal light.

Further, since the second core contains a rare earth element and anamplification band width widening element, it is possible to ensure asufficient wide band width of an optical fiber amplifier. Theamplification band width widening element may be selected from a groupconsisting of Al, Zn, Sn, and La.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration view of a preform preparingapparatus;

FIGS. 2A to 2G are diagrams illustrating sequence steps for preparing apreform according to an embodiment of the present invention;

FIGS. 3A to 3D are diagrams illustrating sequence steps of drawing acovering tube;

FIG. 4A is a transverse sectional view of a quartz reaction tube beforecollapse;

FIG. 4B is a transverse sectional view of a preform;

FIG. 5 is a schematic configuration view of an optical fiber drawingapparatus;

FIG. 6 is a diagram showing a refractive index profile and a mode fieldof the optical amplifying fiber according to the embodiment of thepresent invention;

FIG. 7 is a diagram showing a relationship between a pumping light powerand a signal light output; and

FIG. 8 is a view showing a sectional structure and a refractive indexprofile of an optical amplifying fiber according to another embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a schematic configuration of apreform preparing apparatus usable for preparation of a preform.Reference numeral 2 indicates a lathe for producing a glass rotatablysupporting a quartz reaction tube 4; 6 is a burner reciprocated on thelathe 2 in the longitudinal direction of the quartz reaction tube 4 forheating the quartz reaction tube 4 from the outside thereof; and 8 is atemperature controller for controlling a burning state of the burner 6by adjusting flow rates of O₂ and H₂ or the like fed to the burner 6.

A gas feed pipe 12 is connected to a connector 10 connected to an endportion of the quartz reaction tube 4, and source gases such as SiCl₄and O₂ are fed into the quartz reaction tube 4 via the gas feed pipe 12.Reference numeral 14 indicates feeders for feeding source gases such asSiCl₄ and GeCl₄, and the feed amount of the source gases is controlledby a flow rate of a carrier gas such as O₂ fed via each mass flow meter16.

A solution feed pipe 18 is connected to the connector 10 in parallel tothe gas feed pipe 12, and is also connected to a solution tank 22 via avalve 20. A solution in the solution tank 22 is fed in the quartzreaction tube 4 when the valve 20 is opened. In addition, a connectionportion at which the gas feed pipe 12 and the solution feed pipe 18 areconnected to the quartz reaction tube 4 via the connector 10 is sealedby a known method, to thereby ensure a closed system in the quartzreaction tube 4.

FIGS. 2A to 2G are diagrams illustrating sequence steps of preparing apreform using the preform preparing apparatus shown in FIG. 1. First, aP-F-SiO₂ based clad glass (not shown) is preferably formed in the quartzreaction tube 4 having an outside diameter of 22 mm and an insidediameter of 18 mm by feeding source gases, SiCi₄, POCl₃, and SF₆ in thequartz reaction tube 4. The step of depositing the clad glass may beomitted.

Next, as shown in FIG. 2A, the quartz reaction tube 4 into which sourcegases including SiCl₄ and GeCl₄ and a carrier gas are being fed isrotated and simultaneously heated by the burner 6 from the outsidethereof, so that a fine powder of an oxide glass taken as a first coreis deposited in the quartz reaction tube 4. The fine powder isimmediately vitrified by heating using the burner 6. By repeating thereciprocating motion of the burner 6 by a plurality of times, a firstcore layer 24 made of SiO₂ doped with GeO₂ and having a predeterminedrefractive index and a predetermined thickness is uniformly formed on aninner wall of the quartz reaction tube 4.

The refractive index of the first core layer 24 is set to be higher thanthe refractive index of the quartz reaction tube 4 for obtaining apredetermined relative index difference. The refractive index of thefirst core layer 24 can be adjusted by a composition of the source gasesand the like. For example, the first core layer 24 has a relative indexdifference of about 2.0. Next, while not shown, the feed of the sourcegases is stopped and the quartz reaction tube 4 is heated at a hightemperature by the burner 6, to be thus collapsed slightly. This iscalled an intermediate collapse treatment. After the feed of GeCl₄ ofthe source gases is stopped and the heating temperature of the quartzreaction tube 4 heated by the burner 6 is lowered, the quartz reactiontube 4 is heated at a lower temperature by the burner 6 from the outsidethereof, to deposit a fine powder of an oxide glass made of SiO₂ on thefirst core layer 24.

By repeating the reciprocating motion of the burner 6 by a plurality oftimes, a soot-like second core layer 26 made of SiO₂ is formed on thefirst core layer 24, as shown in FIG. 2B. Here, the wording “soot-like”means a fine powdery or porous state capable of keeping the form of alayer. The lowering of the heating temperature of the quartz reactiontube 4 heated by the burner 6 is to prevent the second core layer 26from being immediately vitrified. After that, as shown in FIG. 2C, theburner 6 is moved to a position near an end portion of the quartzreaction tube 4, and in such a state, the quartz reaction tube 4 islocally heated while being rotated, to form a constricted portion 28having a small diameter at the heated portion. The constricted portion28 is formed at each end portion of the quartz reaction tube 4.

The quartz reaction tube 4 is cooled to a suitable temperature, and asshown in FIG. 2D, a solution feed pipe 18 made of a flexible resin orthe like is introduced into the quartz reaction tube 4 with a leadingend thereof positioned between the constricted portions 28, 28, and asolution fed from the solution tank 22 is injected in the quartzreaction tube 4 at a region between the constricted portions 28, 28. Thesolution thus injected in the quartz reaction tube 4 is impregnated onlyin the soot like second core layer 26. The solution to be fed in thequartz reaction tube 4 contains a rare earth element and Al. In thisembodiment, the solution contains ErCl₃.6H₂O and AlCl₃ as a solute inethanol as a solvent.

The concentration of ErCl₃.6H₂O in the solution is, for example, withina range of 0.001 to 1 wt %. The concentration of the solution forobtaining predetermined dope concentrations of Al₂O₃ and Er in anoptical fiber or a preform can be experimentally determined. Theinjected amount of the solution is, for example, within a range of 5 to20 ml. After the solution feed pipe 18 is retreated, dried N₂ gas is fedin the quartz reaction tube 4 to slowly evaporate alcohol and moisture,and the remaining moisture is sufficiently removed by feeding Cl₂ and O₂in the quartz reaction tube 4 and heating the quartz reaction tube 4 bythe burner 6.

After that, as shown in FIG. 2E, by heating the quartz reaction tube 4by reciprocating the burner 6, the soot-like second core layer 26 isvitrified, to thereby obtain a vitrified second core layer 26′. Arelative index difference of the second core layer 26′ is, for example,0.7%. The quartz reaction tube 4 is then heated at a high temperature bythe burner 6 to perform the intermediate collapse treatment again, andas shown in FIG. 2F, source gases containing SiCl₄ and GeCl₄ and acarrier gas are fed in the quartz reaction tube 4 and at the same timethe quartz reaction tube 4 is heated by the burner 6 from the outsidethereof as in the step shown in FIG. 2A.

A fine powder of an oxide glass taken as a third core layer is depositedon the second core layer 26′, and the fine powder is immediatelyvitrified by heating using the burner 4. By repeating the reciprocatingmotion of the burner 6 for a plurality of times, a third core layer 30made of SiO₂ doped with GeO₂ and having a predetermined refractive indexand a predetermined thickness is uniformly formed on the second corelayer 26′. A relative index difference of the third core layer 30 is,for example, about 2.0%. Finally, as shown in FIG. 2G, the quartzreaction tube 4 is perfectly collapsed until a hollow portion thereofdisappears by further heating the quartz reaction tube 4 at a highertemperature by the burner 6, to obtain a preform 32 a. It is to be notedthat the reason why the intermediate collapse treatment is performedafter each of the steps shown in FIGS. 2A and 2E is to prevent diffusionof Ge, Al and the like between respective core layers as much aspossible. A diameter of the preform 32 a after the perfect collapsetreatment is about 14 mm.

The preform 32 a having a diameter of about 14 mm thus prepared iscovered with a quartz glass tube 34 having an outside diameter of 22 mmas shown in FIG. 3A, and subsequently, as shown in FIG. 3B, it isintegrated with the quartz glass tube 34 by heating, followed by drawingof the covering tube into a diameter of about 14 mm, to obtain a preform32 b. After the covering tube drawing step is repeated for a pluralityof times, the preform 32 b is finally covered with a thick quartz glasstube 36 having an outside diameter of about 26 mm as shown in FIG. 3C,followed by integration therewith by heating, to prepare a preform 32having a diameter of about 14 mm as shown in FIG. 3D.

By repeating the covering tube drawing step for a plurality of times, itbecomes possible to make smaller a relative core diameter and hence tooptimize both a mode field diameter and a cut-off wavelength, and alsoto optimize the inside and outside diameters of the second core portiondoped with Er and hence to improve a conversion efficiency. The secondcore 26′ in this embodiment contains Al at a concentration of about 6 wt%, and Er at a concentration of about 500 ppm.

A transverse cross-section of the quartz reaction tube 4 before theperfect collapse treatment is shown in FIG. 4A, and a transversecross-section of the preform 32 obtained by the step shown in FIG. 3D isshown in FIG. 4B. The preform 32 after the covering tube drawing stepincludes the clad 34 made of SiO₂ and having a relatively low refractiveindex, a first core 36 made of SiO₂ doped with GeO₂ for increasing arefractive index and having a relatively high refractive index, a secondcore 38 made of SiO₂ doped with Er and Al₂O₃ and not doped with GeO₂ anda third core 40 made of SiO₂, doped with GeO₂, for increasing arefractive index and having a relatively high refractive index.

The second core 38 has the refractive index higher than that of the clad34, and the first and third cores 36, 40 have the refractive indexeseach of which is higher than that of the second core 38. The maincomponent SiO₂ of each portion may contain P₂O₅ or the like foradjusting a refractive index. An optical fiber obtained by drawing thepreform 32 shown in FIG. 4B has a transverse cross-section analogous tothat of the preform 32, and it also has each component having the samecomposition as that of the corresponding component of the preform 32.Therefore, each component of the optical fiber is indicated by the samename and reference numeral of the corresponding component of the preform32.

According to the preform preparing process described in this embodiment,since the solution is injected between the constricted portions 28, 28formed in the quartz reaction tube 4 and is impregnated in the secondcore layer 26, the quartz reaction tube 4 is not required to be removedfrom the lathe for impregnation of the solution. In this case, theinjection of the solution between the constricted portions 28, 28 of thequartz reaction tube 4 can be performed from one side of the quartzreaction tube 4, so that it is possible to keep the closed systemincluding the feed system for source gases. This prevents degradation ofa loss characteristic by permeation of impurities in the quartz react-ontube 4. Further, according to the preform preparing process of thisembodiment, since all of the steps of preparing a preform can be carriedout in a state in which the quartz reaction tube 4 is mounted on thelathe 2, there can be eliminated laborious works of, for example,mounting/dismounting the quartz reaction tube 4 to/from the lathe 2.

FIG. 5 is a schematic view of an apparatus for drawing a preform into anoptical fiber. The preform 32 is supported by a preform feeding portion42 and gradually fed downward, and a lower end of the preform 32 isheated to be melted in a heating furnace 44. The preform 32 is drawninto a doped fiber 45 at a lower end portion of the heating furnace 44,and a diameter of the doped fiber 45 is measured by a fiber diametermeasuring unit 46 in a non-contact manner. The doped fiber 45 is thencoated with a ultraviolet (UV) curing epoxy resin by a coating unit 48,followed by curing of the coating by a ultraviolet lamp 50. The dopedfiber 45 coated with the UV curing epoxy resin is wound around a windingdrum 54 via a capstan roller 52 rotating at a controlled speed.

The rotating speed of the capstan roller 52 is feedback-controlled by afiber diameter control unit 56 for keeping constant a diameter of thedoped fiber 45 measured by the fiber diameter measuring unit 46. Thedoped fiber 45 having stable characteristics in the longitudinaldirection, such as, dope concentrations of a rare earth element andAl₂O₃ and a diameter of each component, can be produced from the preform32 using such a drawing apparatus.

In accordance with the above-described production process, the dopedfiber 45 having a triple core structure of the first, second, and thirdcores 36, 38, and 40 is produced. Specifically, the doped fiber 45 hasthe clad 34 made of SiO₂, the first core 36 made of SiO₂ doped withGeO₂, the second core made of SiO₂ doped with Er, Al₂O₃ and not dopedwith GeO₂, and the third core 40 made of SiO₂ doped with GeO₂.

FIG. 6 shows a refractive index profile 60 and a mode field 62 of thedoped fiber 45 obtained in this embodiment. Letting D1, D2, and D3 bediameters of the first, second and third cores 36, 38 and 40respectively, and Δ1, Δ2, and Δ2 be relative index differences of thefirst, second and third cores 36, 38 and 40 respectively, Δ1=about 2%,D1=about 3.0 μm, Δ2=about 0.7%, D2=about 0.8 μm, Δ3=about 2%, andD3=about 0.6 μm. Further, a mode field diameter 64 is 4.4 μm and aconversion efficiency of pumping light into signal light is about 73%.Results of an experiment made for examining conversion efficiencycharacteristics are compared with values of the prior art doped fiberdisclosed in Japanese Patent Laid-open No Hei 5-119222 as shown in Table1 and FIG. 7.

TABLE 1 structure mode field efficiency threshold conversion lossdiameter gradient value efficiency Prior art 20 dB/Km 4.8 μm 70% 7.0 mW64% doped fiber Doped fiber  7 dB/Km 4.4 μm 80% 5.5 mW 73% of theinvention

As is apparent from Table 1, for the doped fiber 45 of this embodiment,the mode field diameter is reduced from the value (4.8 μm) of the priorart doped fiber to 4.4 μm and the conversion efficiency is increasedfrom the value (64%) of the prior art doped fiber to 73%. The thresholdvalue in Table 1 means the minimum value of a pumping light power atwhich a gain of signal light is started to appear.

Referring to FIG. 8, there is shown a sectional structure and arefractive index profile 70 of a doped fiber 64 produced according toanother embodiment of the present invention. The doped fiber 65 of thisembodiment has a configuration that a fourth core 66 and a fifth core 68are provided inside the third core 40 of the doped fiber 45 of the firstembodiment. The fourth core 66 is made of SiO₂ doped with Er, Al₂O₃ andnot doped with GeO₂, and the fifth core 68 is made of SiO₂ doped withGeO₂.

A relative index difference of the fourth core 66 is about 0.7% and arelative index difference of the fifth core 68 is about 2%. In the dopedfiber 65 of this embodiment, an outside diameter of the first core 36positioned at the outermost periphery is preferably to be 2 μm or less.

Although Er is used as a rare earth element in the above-describedembodiments, the present invention is not limited thereto, and forexample, a different rare earth element such as Nd or Yb may be used.Further, TiO₂ may be used as a dopant for increasing a refractive index,and an element such as Zn, Sn or La can be used for widening a bad widthin place of Al.

As described above, the present invention has a meritorious effect ofproviding an optical fiber suitable for realizing an optical fiberamplifier which is wider in wavelength band width and is higher inconversion efficiency of pumping light into signal light.

What is claimed is:
 1. A process of producing an optical amplifyingfiber, comprising the steps of in the following order: (a) forming afirst core layer mainly made of SiO₂ doped with GeO₂ or TiO₂ on an innersurface of a quartz reaction tube by chemical vapor deposition, saidfirst core layer having a first refractive index; (b) forming asoot-like second core layer mainly made of SiO₂ on said first core layerby chemical vapor deposition; (c) impregnating said second core layerwith a solution containing a solvent, a rare earth element and at leastone element selected from the group consisting of Al, Zn, Sn and La; (d)evaporating the solvent in the solution impregnated in said second corelayer; (e) heating said second core layer to vitrify said second corelayer, said second core layer having a second refractive index lowerthan the first refractive index; (f) forming a third core layer mainlymade of SiO₂ doped with GeO₂ or TiO₂ on said second core layer bychemical vapor deposition, said third core layer having a thirdrefractive index greater than the second refractive index; (g)substantially uniformly collapsing said quartz reaction tube by heatingto form a preform; and (h) melting and spinning said preform to therebyproduce an optical amplifying fiber having first, second and third coreshaving compositions and refractive indexes corresponding respectively tothose of said first, second and third core layers such that therefractive index of the second core is lower than the refractive indexof the first core and the refractive index of the third core is greaterthan the refractive index of the second core, said third core having anouter diameter of 1 μm or less and being the inner most core of saidfiber, the concentration of said at least one element in said secondcore being 4 wt % or more.
 2. A process of producing an opticalamplifying fiber according to claim 1, wherein the solution impregnatedin said second core layer contains Er and Al.
 3. A process of producingan optical amplifying fiber according to claim 1, further comprisingsteps of: between steps (a) and (b), performing an intermediate collapsetreatment on the quartz reaction tube by heating the quartz reactiontube to thereby make smaller an outside diameter of the quartz reactiontube; and between steps (e) and (f), performing an intermediate collapsetreatment on the quartz reaction tube by heating the quartz reactiontube to thereby make smaller the outside diameter of the quartz reactiontube.
 4. A process of producing an optical amplifying fiber according toclaim 1, further comprising the steps of covering the preform collapsedin step (g) with a new quartz glass tube, integrating the preform withthe new quartz glass tube by heating, and drawing the integrated preformwith the quartz glass tube.
 5. A process of producing an opticalamplifying fiber according to claim 1, wherein the quartz reaction tubehas a fourth refractive index, the first refractive index being greaterthan the fourth refractive index.
 6. A process of producing an opticalamplifying fiber according to claim 1, wherein the quartz reaction tubehas end portions and an intermediate portion, and between steps (b) and(c), the end portions of the quartz reaction tube are constricted withrespect to the intermediate portion.
 7. A process of producing anoptical amplifying fiber, comprising: (a) forming a first core layercontaining SiO₂ doped with GeO₂ or TiO₂ on an inner surface of areaction tube, said first core layer having a first refractive index;(b) forming a second core layer containing SiO₂ on said first corelayer; (c) impregnating said second core layer with a solutioncontaining a solvent, a rare earth element and at least one elementselected from the group consisting of Al, Zn, Sn and La; (e) heatingsaid second core layer to vitrify said second core layer, said secondcore layer having a second refractive index lower than the firstrefractive index; (f) forming a third core layer containing SiO₂ dopedwith GeO₂ or TiO₂ on said second core layer, said third core layerhaving a third refractive index greater than the second refractiveindex; (g) collapsing said reaction tube by heating to form a preform;and (h) melting and spinning said preform to thereby produce an opticalamplifying fiber having first, second and third cores havingcompositions and refractive indexes corresponding respectively to thoseof said first, second and third core layers such that the refractiveindex of the second core is lower than the refractive index of the firstcore and the refractive index of the third core is greater than therefractive index of the second core, said third core having an outerdiameter of 1 μm or less and being the inner most core of said fiber,the concentration of said at least one element in said second core being4 wt % or more.